1. Field of the Invention
This invention relates to medical devices and implantable techniques for treating patients by an implantable electrical transducer capable of movement, and more particularly to such techniques in which the transducer is powered by an implantable and rechargeable capacitive energy storage unit.
2. Description of the Related Art
Batteries have been used in the past to power implanted medical devices that include electrical transducers, such as motors that power pumps for infusing drugs and other medication. The batteries usually take the form of nickel cadmium batteries that typically are recharged via a radio frequency link. With such a system, it requires a significant time period, such as two hours, to recharge the batteries.
Capacitors have been used to a limited extent to power implantable devices in the past. For example, U.S. Pat. No. 4,408,607 (Maurer, issued Oct. 11, 1983) describes a capacitive energy source and circuitry for powering medical apparatus.
Other methods of powering implantable medical devices have also been tried. For example, U.S. Pat. No. 4,102,344 (Conway et al.) discloses an implantable device that is powered by an external high-frequency transmitter. U.S. Pat. No. 5,312,439 (Loeb) discloses an implantable device having an electrolytic capacitive storage electrode. U.S. Pat. No. 5,324,316 (Schulman et al.) discloses an implantable microstimulator which has one or more electrodes immersed in body fluids. The capacitor formed by the body fluids and the electrodes stores one hundred microcoulombs of charge. U.S. Pat. No. 5,358,514 (Schulman) discloses an implantable medical stimulator including a capacitor 20 without describing the capacitive rating or volumetric size of the capacitor. U.S. Pat. No. 5,405,367 (Schulman) discloses the use of a capacitor 20 on the order of 2-30 microfarads. U.S. Pat. No. 3,258,013 (Druz) discloses a defibrillator which is operable from a wholly self-contained battery power source 10. The Druz Patent also discloses shunt condensers or capacitors 36 and 37 which are part of a two-sectioned lumped constant delay line. A delay line discharge pulsing circuit is employed as an energy storage device. Capacitors 36 and 37 are 20 microfarads each. U.S. Pat. No. 3,209,081 (Ducote) discloses an implant with transistors powered from a radio receiver. Voltage is applied to a capacitor 20 as long as the power transmitter is functioning. U.S. Pat. No. 4,408,608 (Daly) discloses an implantable stimulator using a capacitor C1 that tunes a coil L1. U.S. Pat. No. 5,591,217 (Barreras issued Jan. 7, 1997) discloses an implantable stimulator with a capacitive power source having a capacitive rating of at least 0.1 farads or higher.
Although the foregoing techniques have recognized some value in the use of capacitors in medical devices, they either have used conventional capacitors which are inferior to nickel cadmium batteries in significant respects, have limited application of capacitive power sources to electrical stimulation.